Drawing texture on surface
Your grass texture is repeated (tiled) across the surface. If try to texture paint "over", you are actually painting into that texture so the drawing will repeat too.
If you want to blend the gravel painting over your tiled grass texture, you can do it by several methods, for example:


Add grass borders to your gravel texture (gravel fading into the grass), cut the mesh where you want to make a gravel surface and map that "gravel to grass blend texture". Or you can make a segment with this texture mapped and use Array and Curve modifiers to create a gravel path as in the screenshot below... and then merge this path model to the grass geometry. This method creates some seams between the textures so but it can be almost negligible if the transition is painted well. The seams are often covered by some grass / foliage models.


Creating a gravel path using one gravel segment and Array + Curve modifiers

Merged into grass model:

 2. You can cut the area you plan to cover with the gravel, bake the grass texture from this area to a new texture and paint the gravel into it. Search for a Texture Baking tutorial if you don't know how to do it. This method is usable only for small areas because the texture needs to cover all the gravel area so it can become too large for using it in game.

 3. The game engine you want to use may be able to blend textures nativelly (the blending can be for example defined by Vertex painting) ...so take a look in the documentation to see which methods does it support and look for a tutorial.
This is some Vertex Paint in Unity https://www.youtube.com/watch?v=HDXwSEUYrbI